Question

我遇到了Calendar的after方法的奇怪错误。下面的代码占用了当前时间，并且应该在明天的同一时间返回。使用当前时间运行代码时会发生错误。有什么想法正在发生什么？

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Vector;

public class NextDateTest {

    public static void main(String[] args) {

        Vector<Object> setup = new Vector<Object>();
        Calendar dt = Calendar.getInstance();
        SimpleDateFormat hour = new SimpleDateFormat("HH");
        SimpleDateFormat minute = new SimpleDateFormat("mm");

        setup.add(hour.format(dt.getTime()));
        setup.add(minute.format(dt.getTime()));

        for(int a=0; a<11; a++){
            dateTest(setup);
        }

    }

    static void dateTest(Vector<Object> vec){
        Calendar dt = Calendar.getInstance();
        SimpleDateFormat format = new SimpleDateFormat("MM/dd/yyyy HH:mm");
        System.out.println("Old time:" + format.format(dt.getTime()));

        dt.set(Calendar.HOUR_OF_DAY, Integer.valueOf((String) vec.elementAt(0)));
        dt.set(Calendar.MINUTE, Integer.valueOf((String) vec.elementAt(1)));
        System.out.println(Calendar.getInstance().after(dt));
        if(Calendar.getInstance().after(dt)){
            dt.add(Calendar.DAY_OF_YEAR, +1);
        }           
        System.out.println("New time:" + format.format(dt.getTime()));  
    }

}

结果：

Old time:12/19/2011 10:38
true
New time:12/20/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/19/2011 10:38
Old time:12/19/2011 10:38
true
New time:12/20/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/19/2011 10:38
Old time:12/19/2011 10:38
true
New time:12/20/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/19/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/20/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/19/2011 10:38
Old time:12/19/2011 10:38
true
New time:12/20/2011 10:38
Old time:12/19/2011 10:38
false
New time:12/19/2011 10:38
Old time:12/19/2011 10:38
true
New time:12/20/2011 10:38

Answer 1

这不是一个错误。 Calendar.after(...)执行严格不等式，Calendar仅存储毫秒精度。（就此而言，我甚至不确定它是否保证在所有系统上精确到毫秒;我相信有些系统不会让软件访问那么小的时间增量。但是毫秒是典型的。）因此，如果dateTest(...)对Calendar.getInstance()的两次调用在同一毫秒内发生，那么Calendar.after(...)将返回false，dt.add(Calendar.DAY_OF_YEAR, +1)将不会进行。

方法后的Java Calendar返回不一致的结果 - 任何想法？

1 个答案: